Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic waves and acoustic waves to electrical signal using inverse and direct piezo-electric effects. Acoustic transducers generally include acoustic resonators, such as thin film bulk acoustic resonators (FBARs), surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. FBAR devices, in particular, generate acoustic waves that can propagate in all possible lateral directions when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The laterally propagating modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
Filters based on FBAR technology provide a comparatively low in-band insertion loss due to the comparatively high quality factor (Q-factor) of FBAR devices. FBAR-based filters are often employed in cellular or mobile telephones that can operate in multiple frequency bands. In such devices, it is important that a filter intended to pass one particular frequency band (“passband”) should have a high level of attenuation at other nearby frequency bands which contain signals that should be rejected. Specifically, there may be one or more frequencies or frequency bands near the passband which contain signals at relatively high amplitudes that should be rejected by the filter. In such cases, it would be beneficial to be able to increase the filter's rejection characteristics at those particular frequencies or frequency bands, even if the rejection at other frequencies or frequency bands does not receive the same level of rejection.
Other types of filters are based on FBAR technology, including a stacked bulk acoustic resonator (SBAR), also referred to as a double bulk acoustic resonator (DBAR), and a coupled resonator filter (CRF). The DBAR includes two layers of piezoelectric materials between three electrodes in a single stack, forming a single resonant cavity. That is, a first layer of piezoelectric material is formed between a first (bottom) electrode and a second (middle) electrode, and a second layer of piezoelectric material is formed between the second (middle) electrode and a third (top) electrode. Generally, the DBAR device allows reduction of the area of a single bulk acoustic resonator device by about half.
A CRF comprises a coupling structure disposed between two vertically stacked FBARs. The CRF combines the acoustic action of the two FBARs and provides a bandpass filter transfer function. For a given acoustic stack, the CRF has two fundamental resonance modes, a symmetric mode and an anti-symmetric mode, of different frequencies. The degree of difference in the frequencies of the modes depends, inter alia, on the degree or strength of the coupling between the two FBARs of the CRF. When the degree of coupling between the two FBARs is too great (over-coupled), the passband is unacceptably wide, and an unacceptable “swag” or “dip” in the center of the passband results, as does an attendant unacceptably high insertion loss in the center of the passband. When the degree of coupling between the FBARs is too low (under-coupled), the passband of the CRF is too narrow.
All FBARs and filters based on FBARs have an active region. The active region of a CRF, for example, comprises the region of overlap of the top FBAR, the coupling structure, and the bottom FBAR. Generally, it is desirable to confine the acoustic energy of certain desired acoustic modes within the active region. As should be appreciated by one of ordinary skill in the art, at the boundaries of the active region, reflection of desired modes can result in mode conversion into spurious/undesired modes, and loss of acoustic energy over a desired frequency range (e.g., the passband of the CRF).
In FBAR devices, mitigation of acoustic losses at the boundaries and the resultant mode confinement in the active region of the FBAR (the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode) has been effected through various methods. Notably, frames are provided along one or more sides of the FBARs. The frames create an acoustic impedance mismatch that reduces losses by reflecting desired modes back to the active area of the resonator, thus improving the confinement of desired modes within the active region of the FBAR.
While the incorporation of frames has resulted in improved mode confinement and attendant improvement in the Q-factor of the FBAR, direct application of known frame elements has not resulted in significant improvement in mode confinement and Q-factor of conventional DBARs and CRFs. Better acoustic energy confinement, as well as further improvements in FBAR Q-factor due to the better acoustic energy confinement, are needed for increased efficiency of FBARs, DBARs and CRFs.